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268 lines
9.7 KiB
C++
268 lines
9.7 KiB
C++
// SPDX-License-Identifier: GPL-2.0-or-later
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#ifndef SAFE_OP_HPP_
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#define SAFE_OP_HPP_
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#include <limits>
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#include <stdexcept>
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#ifdef _MSC_VER
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#include <Intsafe.h>
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#endif
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/*!
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* @brief Arithmetic operations with overflow checks
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*/
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namespace Safe {
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/*!
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* @brief Helper functions for providing integer overflow checks.
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*
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* This namespace contains internal helper functions fallback_$op_overflow
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* and builtin_$op_overflow (where $op is an arithmetic operation like add,
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* subtract, etc.). Both provide the following interface:
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*
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* bool fallback/builtin_$op_overflow(T first, T second, T& result);
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*
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* where T is an integer type.
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*
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* Each function performs checks whether first $op second can be safely
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* performed without overflows. If yes, the result is saved in result and
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* false is returned. Otherwise true is returned and the contents of result
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* are unspecified.
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*
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* fallback_$op_overflow implements a portable but slower overflow check.
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* builtin_$op_overflow uses compiler builtins (when available) and should
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* be faster. As builtins are not available for all types,
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* builtin_$op_overflow falls back to fallback_$op_overflow when no builtin
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* is available.
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*/
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namespace Internal {
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/*!
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* @brief Helper struct to determine whether a type is signed or unsigned
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*
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* This struct is a backport of std::is_signed from C++11. It has a public
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* enum with the property VALUE which is true when the type is signed or
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* false if it is unsigned.
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*/
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template <typename T>
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struct is_signed {
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enum { VALUE = static_cast<T>(-1) < static_cast<T>(0) };
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};
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/*!
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* @brief Helper struct for SFINAE, from C++11
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* This struct has a public typedef called type typedef'd to T if B is
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* true. Otherwise there is no typedef.
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*/
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template <bool B, class T = void>
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struct enable_if {};
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/*!
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* @brief Specialization of enable_if for the case B == true
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*/
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template <class T>
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struct enable_if<true, T> {
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using type = T;
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};
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/*!
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* @brief Check the addition of two numbers for overflows for signed
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* integer types larger than int or with the same size as int.
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*
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* This function performs a check if summand_1 + summand_2 would
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* overflow and returns true in that case. If no overflow occurs,
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* the sum is saved in result and false is returned.
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*
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* @return true on overflow, false on no overflow
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*
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* @param[in] summand_1, summand_2 The summands with are added
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* @param[out] result Result of the addition, only populated when no
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* overflow occurs.
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*
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* Further information:
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* https://wiki.sei.cmu.edu/confluence/display/c/INT32-C.+Ensure+that+operations+on+signed+integers+do+not+result+in+overflow
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*/
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template <typename T>
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typename enable_if<is_signed<T>::VALUE && sizeof(T) >= sizeof(int), bool>::type fallback_add_overflow(T summand_1,
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T summand_2,
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T& result) {
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if (((summand_2 >= 0) && (summand_1 > std::numeric_limits<T>::max() - summand_2)) ||
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((summand_2 < 0) && (summand_1 < std::numeric_limits<T>::min() - summand_2))) {
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return true;
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} else {
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result = summand_1 + summand_2;
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return false;
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}
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}
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/*!
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* @brief Check the addition of two numbers for overflows for signed
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* integer types smaller than int.
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*
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* This function adds summand_1 and summand_2 exploiting integer
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* promotion rules, thereby not causing undefined behavior. The
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* result is checked against the limits of T and true is returned if
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* they are exceeded. Otherwise the sum is saved in result and false
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* is returned.
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*
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* @return true on overflow, false on no overflow
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*
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* @param[in] summand_1, summand_2 The summands with are added
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* @param[out] result Result of the addition, only populated when no
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* overflow occurs.
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*
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* Further information:
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* https://wiki.sei.cmu.edu/confluence/display/c/INT02-C.+Understand+integer+conversion+rules
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*/
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template <typename T>
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typename enable_if<is_signed<T>::VALUE && sizeof(T) < sizeof(int), bool>::type fallback_add_overflow(T summand_1,
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T summand_2,
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T& result) {
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const int res = summand_1 + summand_2;
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if ((res > std::numeric_limits<T>::max()) || (res < std::numeric_limits<T>::min())) {
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return true;
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} else {
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result = static_cast<T>(res);
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return false;
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}
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}
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/*!
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* @brief Check the addition of two numbers for overflows for unsigned
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* integer types.
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*
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* This function adds summand_1 and summand_2 and checks after that if
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* the operation overflowed. Since these are unsigned integers, no
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* undefined behavior is invoked.
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*
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* @return true on overflow, false on no overflow
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*
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* @param[in] summand_1, summand_2 The summands with are added
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* @param[out] result Result of the addition
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*
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* Further information:
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* https://wiki.sei.cmu.edu/confluence/display/c/INT30-C.+Ensure+that+unsigned+integer+operations+do+not+wrap
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*/
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template <typename T>
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typename enable_if<!is_signed<T>::VALUE, bool>::type fallback_add_overflow(T summand_1, T summand_2, T& result) {
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result = summand_1 + summand_2;
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return result < summand_1;
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}
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/*!
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* @brief Overflow addition check using compiler intrinsics.
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*
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* This function behaves exactly like fallback_add_overflow() but it
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* relies on compiler intrinsics instead. This version should be faster
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* than the fallback version as it can fully utilize available CPU
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* instructions & the compiler's diagnostic.
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*
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* However, as some compilers don't provide intrinsics for certain
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* types, the default implementation is the version from fallback.
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*
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* This function is fully specialized for each compiler.
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*/
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template <typename T>
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bool builtin_add_overflow(T summand_1, T summand_2, T& result) {
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return fallback_add_overflow(summand_1, summand_2, result);
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}
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#if defined(__GNUC__) || defined(__clang__)
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#if __GNUC__ >= 5 || __clang_major__ >= 3
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/*!
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* This macro pastes a specialization of builtin_add_overflow using gcc's &
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* clang's __builtin_(s/u)add(l)(l)_overlow()
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*
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* The add function is implemented by forwarding the parameters to the intrinsic
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* and returning its value.
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*
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* The intrinsics are documented here:
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* https://gcc.gnu.org/onlinedocs/gcc/Integer-Overflow-Builtins.html#Integer-Overflow-Builtins
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*/
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#define SPECIALIZE_builtin_add_overflow(type, builtin_name) \
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/* Full specialization of builtin_add_overflow for type using the */ \
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/* builtin_name intrinsic */ \
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template <> \
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inline bool builtin_add_overflow<type>(type summand_1, type summand_2, type & result) { \
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return builtin_name(summand_1, summand_2, &result); \
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}
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SPECIALIZE_builtin_add_overflow(int, __builtin_sadd_overflow);
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SPECIALIZE_builtin_add_overflow(long, __builtin_saddl_overflow);
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SPECIALIZE_builtin_add_overflow(long long, __builtin_saddll_overflow);
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SPECIALIZE_builtin_add_overflow(unsigned int, __builtin_uadd_overflow);
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SPECIALIZE_builtin_add_overflow(unsigned long, __builtin_uaddl_overflow);
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SPECIALIZE_builtin_add_overflow(unsigned long long, __builtin_uaddll_overflow);
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#undef SPECIALIZE_builtin_add_overflow
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#endif // __GNUC__ >= 5 || __clang_major >= 3
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#endif
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} // namespace Internal
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/*!
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* @brief Safe addition, throws an exception on overflow.
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*
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* This function returns the result of summand_1 and summand_2 only when the
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* operation would not overflow, otherwise an exception of type
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* std::overflow_error is thrown.
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*
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* @param[in] summand_1, summand_2 summands to be summed up
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* @return the sum of summand_1 and summand_2
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* @throws std::overflow_error if the addition would overflow
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*
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* This function utilizes compiler builtins when available and should have a
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* very small performance hit then. When builtins are unavailable, a more
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* extensive check is required.
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*
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* Builtins are available for the following configurations:
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* - GCC/Clang for signed and unsigned int, long and long long (not char & short)
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* - MSVC for unsigned int, long and long long
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*/
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template <typename T>
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T add(T summand_1, T summand_2) {
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T res = 0;
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if (Internal::builtin_add_overflow(summand_1, summand_2, res)) {
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throw std::overflow_error("Overflow in addition");
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}
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return res;
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}
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/*!
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* @brief Calculates the absolute value of a number without producing
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* negative values.
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*
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* The "standard" implementation of `abs(num)` (`num < 0 ? -num : num`)
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* produces negative values when `num` is the smallest negative number. This
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* is caused by `-1 * INTMAX = INTMIN + 1`, i.e. the real result of
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* `abs(INTMIN)` overflows the integer type and results in `INTMIN` again
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* (this is not guaranteed as it invokes undefined behavior).
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*
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* This function does not exhibit this behavior, it returns
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* `std::numeric_limits<T>::max()` when the input is
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* `std::numeric_limits<T>::min()`. The downside of this is that two
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* negative values produce the same absolute value:
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* `std::numeric_limits<T>::min()` and `std::numeric_limits<T>::min() + 1`.
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*
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* @tparam T a signed integer type
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* @param[in] num The number which absolute value should be computed.
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* @throws Never throws an exception.
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* @return The absolute value of `num` or `std::numeric_limits<T>::max()`
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* when `num == std::numeric_limits<T>::min()`.
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*/
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template <typename T>
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typename Internal::enable_if<Internal::is_signed<T>::VALUE, T>::type abs(T num) throw() {
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if (num == std::numeric_limits<T>::min()) {
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return std::numeric_limits<T>::max();
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}
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return num < 0 ? -num : num;
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}
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} // namespace Safe
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#endif // SAFE_OP_HPP_
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